It is known that the efficiency of a combustion turbine engine improves as the firing temperature of the combustion gas is increased. As the firing temperatures increase, the high temperature durability of components of the turbine must increase correspondingly. Although nickel and cobalt based superalloy materials may be used for components in the hot gas flow path, such as combustor transition pieces and turbine rotating and stationary blades, even these superalloy materials are not capable of surviving long term operation at temperatures that sometimes can exceed 1,400 degrees C. or more.
In many applications, a metal substrate is coated with a ceramic insulating material, such as a thermal barrier coating (TBC), to reduce the service temperature of the underlying metal and to reduce the magnitude of temperature transients to which the metal is exposed. TBCs have played a substantial role in realizing improvements in turbine efficiency. However, one basic physical reality that cannot be overlooked is that the thermal barrier coating will only protect the substrate so long as the coating remains substantially intact on the surface of a given component through the life of that component.
High stresses that may develop due to high velocity ballistic impacts by foreign objects and/or differential thermal expansion can lead to damage and even total removal of the TBC (spallation) from the component. It is known to control a roughness parameter of a surface in order to improve the adhesion of an overlying thermal barrier coating. U.S. Pat. No. 5,419,971 describes a laser ablation process where removal of material by direct vaporization (e.g., without melting of material) is purportedly used to form three-dimensional features at the surface being irradiated. See col. 6, line 3. Thus, such features are generally limited to patterns at the surface being irradiated (e.g., do not generally form structures extending outside the surface) and thus processes that can provide improved structural formations conducive to enhanced adhesion are needed.